Borderline metals

The ionization of halides of hard or borderline metal ions is enhanced by decreased donor properties of the halide ion and by increased donor properties of the neutral donor. Thus cobalt (II) iodide and cobalt (II) bromide are completely ionized in dimethyl sulfoxide43) (see Sect. 3.2.). 

For borderline metals and/or borderline ligands, the order is determined by other factors such as dehydration (removal of hydrate water), steric effects, etc. 

Potentially toxic compounds in the subsurface, such as Cd ", Pb ", or Hg ", which are generally found in very low concentrations, are considered soft cations (Buffle 1988). These ions have strong affinity to intermediate and soft ligands and therefore bond to them covalently. Borderline cations, which embrace transition metals like Cu and Ztfexhibit affinity for the soft cations as well as for alkaline-earth compounds. The order of donor atom affinity for soft metals is O < N < S. Functional groups present in subsurface organic matter that show affinity for soft and borderline metals are shown in Table 14.2. 

Thioacetamide, MeC(=S)NH2 (taa), is isoelectronic with thiourea which it resembles by acting as a unidentate S-donor ligand. Thiobenzamide, PhC(=S)NH2 (tba), behaves similarly. Both thioamides form stable complexes with (b) class and borderline metals. Thioacetamide forms the tetrahedral complexes [MX2(taa)2] (M = Fe, Co, Zn X = Cl, Br, I, NCS).47,153-157 The v(M—S) frequency is rather low, ranging from 255 to 230cm, .14s The Mossbauer spectra of FeX2(taa)2 PC = Cl, Br) are consistent with a tetrahedral configuration.158... 

The / -mercapto thioesters (69 R = Et, Pr1) and the o-mercaptobenzoic acid thioesters (68 R = Et, Pr1, Pe ) form complexes with Cu1, Ni11 and Co111.255,258 Since several (b) class and borderline metals form more stable complexes with the S-esters (68) than with the O-esters (67), it is likely that the S-esters behave as S,S-bidentates. 

Although this view is oversimplified and borderline metal carbene complexes have been isolated, this approach is convenient for discussing the activity of metal carbene species in the ring-opening metathesis polymerisation of cycloolefins. Calculations have predicted [81,82] and recent results have shown [83] that, in some systems, metal alkylidene reactivity is competitive with metal carbene reactivity, i.e. olefin metathesis is competitive with olefin cyclopropanation. 

Borderline metal ions form an intermediate group that does not exhibit strong preferences for some ligands over others. 

Most of the first row transition elements, and Cd, exhibit intermediate bonding preferences, sometimes occurring in chelation matrices containing both oxygen and nitrogen or sulfur ligands. Because Cd has been the subject of considerable toxicological study in marine systems, Cd++ will be used as an example of a borderline metal ion. 

Metal ions favoring acetate appear at the beginning of the O N scale and those favoring ammonia at the end. Of the first 13 metal ions, 11 are hard, bnt in this gronp there is one soft (T1+) and one borderline (Pb +) metal ion. Next a group of four borderline metal ions also contains one soft metal ion (Cd +). The scale ends with four soft metal ions, but also with the hard proton. 

Several B metals, the borderline metal Co, and all metalloid metals can form element-carbon bonds that are stable in water. Metallorganic compounds of A ions and of borderline ions (exception Co) hydrolyze in contact with water. Methylation and alkylation are common reactions in biological systems they may also proceed abiotically and generate methyl- or alkyl-element compounds. Examples include selenide, selenoamino acids, methylarsenic acid (MM A A), and dimethylarsenic acid (DM A A) ... 

---